MATEC Web of Conferences 262, 04005 (2019) https://doi.org/10.1051/matecconf/201926204005 KRYNICA 2018 Water Content–Density Criteria for Determining Geomembrane–Fly Ash Interface Shear Strength Katarzyna Zabielska-Adamska1,* 1 Bialystok Technical University, Faculty of Civil and Environmental Engineering, Wiejska Street 45E, 15-351 Bialystok, Poland Abstract. The aim of the present paper was to determine shear strength at the interface between fly ash, as a material underlying artificial sealing layer of storage yards, and HDPE geomembranes. The fly ash was compacted at moisture contents ranging over optimum water contents ± 5% using the standard Proctor test. The shear strength and interaction tests were conducted in classic direct shear apparatus with a cylindrical shear box. For interface strength tests the bottom box frame was equipped with a polycarbonate platen, which enabled geomembrane fixing. The shear strength of the interface contact of fly ash–smooth HDPE geomembrane did not greatly depend on moisture at compaction; however, it was important for textured geomembrane. The lowest interface strength was obtained at the highest moisture w= wopt + 5%, and the greatest values at moistures w ≥ wopt, for both geomembranes. 1 Introduction of compacted non-cohesive fly ash and fly ash/bottom ash mixture are dependent on moisture content during Mineral soil liners and covers are most often single, compaction, w, as are properties of cohesive mineral double or multilayer complex sealing, consisting of soils [14]. Consequently, different values are obtained compacted cohesive soil layers, with coefficient of for w on either side of the wopt on a compaction curve, –9 permeability, k, lower than 10 m/s, characterised by a for the same dry densities, ρd. Thus, w at compaction long-lasting ability to bond and interrupt movement of would be expected to affect interface shear strength. The chemical compounds from landfill leachate as well as aim of the present study was to identify the dependence artificial layers – geomembranes [1-4]. Leakage through of interface contact parameters on fly ash moisture the damaged geomembrane is minimised by placing a content at compaction. The tested fly ash was compacted mineral liner beneath the membrane [5-6]. In recent by the Standard (SP) Proctor method at various w years, there has been much research on utilizing fly ash ranging over wopt ± 5%, with reference to specific fly ash for mineral sealing layers, due to its chemical, physical properties. and mechanical properties. The hydraulic conductivity of fly ash built-in mineral liners ranges from 10–6 to 10–9 m/s, and depends on fly ash compaction and calcium 2 Background oxide content among other properties. Fly ash retains Interface contact is most often tested in modified direct various contaminants including heavy metals [7-9]. Fly shear apparatus or torsional-ring shear apparatus. For ash water-permeability decreases with time, and using interface shear strength the generalised classic Coulomb waste leachate for permeability tests does not affect or condition assumes the form: slightly decreases it [8, 10]. Fly ash hydraulic conductivity remained constant after adding alkaline or τf = σ · tanδ + ca (1) neutral non-organic solution, and increased with acidity [9, 11]. Shear strength of compacted fly ash and its where τf is the soil–geomembrane contact resistance at bearing capacity are significantly greater than for the moment of shearing, σ the shear stress to destruction mineral soils of similar grain size, at similar values of plane (normal stress), ca the adhesion and δ the interface consolidation [12, 13]. friction angle. In the present study the shear strength interface Shear strength test results of soil–geomembrane between compacted fly ash, as a material underlying contact are usually presented for the peak strength artificial sealing layers of storage yards, and HDPE (maximum contact resistance at the moment of shearing) geomembranes with smooth or textured textures, is and residual strength (steady-state value of shearing shown. In previous research it has been frequent practice resistance). In order to reach the steady-state, a large to present the shear strength values of interface contact shear displacement may be required, so a torsional-ring only for mineral soil compacted at water content closed shear apparatus or shear box larger than used in classic to optimum. The permeability and mechanical properties tests is necessary. It is considered that test results of the * Corresponding author: [email protected] © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). MATEC Web of Conferences 262, 04005 (2019) https://doi.org/10.1051/matecconf/201926204005 KRYNICA 2018 interface shear strength, presented as peak values at 3 Laboratory tests maximal shearing resistance, should be used in stability analysis of waste disposal site base. Residual strength values (at a steady-state) are applied when calculations 3.1 The materials of surface sealing layer stability are done for the sake of Laboratory tests were performed on samples of power predicted displacement values in multi-layer sealing industry waste from bituminous coal combustion in [15]. Thermal-Electric Power Station in Bialystok (Poland), Dove et al. [16] tested dependence of non-cohesive stored in a dry storage yard. This waste is referred to as soil particle shape on speed of mobilising peak values or fly ash since it had only a vestige of bottom ash. Fly ash, reaching steady state of shear strength for non-dilative regardless of origin, is fundamentally aluminosilicate interface systems. The displacement requirement to glass and may consist of clay minerals mullite or reach a steady state decreased as particle angularity kaolinite, among others. Scanning electron micrograph increased, but peak values were obtained more slowly. (SEM) images of fly ash particles (Fig. 1) show a They concluded that rounded and sub-rounded grains complex structure of different glassy spheres, spheroids may need a large-displacement shearing test to reach a and agglomerates. steady state. Concluding with these statements to present study, spherical fly ash grains may require less time for a) peak value mobilization than mineral soil grains. A steady state should be reached quickly, as highly brittle fly ash grains are damaged in the process and become more angular during shearing and interface shearing. The dependence between interface contact parameters and w of mineral clay layers at compaction was found in tests after slope-stability failure in hazardous waste landfill at the Kettleman Hills [17]. Seed and Boulanger [18] attempted to determine interface shear strength, between a compacted clay mineral layer and a smooth HDPE geomembrane, depending on moisture content at compaction by the SP method. Tests were carried out in direct shear apparatus with a small box, under unconsolidated and undrained conditions. They found that interface shear strength b) decreased with increased moisture content at compaction. Differences in strength values versus moisture content were reduced when samples were pre- soaked under light surcharge before shearing. On the basis of tests by other researchers, Stark and Poeppel [15] described the impact of clay water content at compaction on interface strength, which decreased with increased normal stress, nearly completely disappearing at σ = 48 kPa, for peak as well as residual values. Interesting tests on clay soils or Ottawa sand contact with smooth geomembranes were done in modified direct shear apparatus, fitted with a miniature pore pressure transducer to measure pore-pressure changes close to soil–geomembrane contact [19, 20]. Soil samples were compacted at various w to the same bulk Fig. 1. Scanning electron micrographs (SEM): (a) a general density and tested under saturated and unsaturated view of tested fly ash grains with visible smooth and vesicular conditions. They reported that the suction nearby the aluminosilicate cenospheres and (b) some crumbled (after geomembrane surface took part in the mobilisation of compaction) fly ash grains. interface shear strength and caused effective normal stresses to be higher than total normal stresses, All tests were carried out on the same fly ash increasing interface shear strength values in the low shipment, which graining was similar to sandy silt. The normal stress range. The shear-strength equation for median diameter D50 was 0.07 mm. The uniformity unsaturated soils was satisfactory used for describing coefficient, CU, equalled 4.00 rated it as a uniform soil interface shear strength at normal stresses lower than 20 (uni-fraction). The curvature coefficient, CC = 1.26, kPa. indicated fly ash was relatively well graded for compaction. The average density of solid particles was very low about 2.09 ± 0.03 Mg/m3. Compaction parameters were obtained by means of the SP method, thus each point of the compaction curve was determined for separately prepared specimen. Fly ash re-compaction 2 MATEC Web of Conferences 262, 04005 (2019) https://doi.org/10.1051/matecconf/201926204005 KRYNICA 2018 causes partial crumbling of dynamically rammed grains Two HDPE geomembranes, one smooth and one and changes physical properties, giving incorrect textured, were used in the tests, both with a core estimation of compaction effects. During compaction, thickness of 1.5 mm. The textured geomembrane smaller grains stuff greater spherical (empty) grains that roughness (Fig. 3), co-extruded during manufacture, had have been partly crushed, so fly ash dry density and an average height on the outside surface of 0.5 mm compaction are better [21]. The maximum dry density, (point convexities irregularly placed on the surface 0.5–4 3 ρd max, was 0.984 Mg/m and optimum water contents, mm apart). wopt, 46.0% by the SP method. Shear strength tests were performed in direct shear apparatus, under the same conditions as interface contact tests, on fly ash compacted at the range wopt ± 5% (Fig.